This invention relates in general to devices for treating articles prior to coating them for printing on them.
The invention relates to an apparatus for changing the surface tension of cup-like containers of round, oval or other form without corners, by corona treatment. A cup-like container rests with its inner side wall on a mandrel rotatably mounted on an axle and having a rotation-symmetrical cross-section of a diameter which is equal to or smaller than the smallest diameter of the cup-like container, and the cup-like container is effected by at least one drive wheel which is pressed onto the outer side of the wall of the cup-like container to roll off on the mandrel.
As is known, the surfaces of cup-like containers, for example of plastic, which are to be perfected e.g. by printing, have too low a surface tension, so that the adhesion of the printing inks is insufficient.
A known method for increasing the surface tension is so-called corona treatment. It is applied, among others, also for rotation-symmetrical containers, in that the container is placed on a rotating counter-electrode whose dimensions match the inner dimensions of the container to be treated, the electrode being stationary and forming a gap with the container surface to be treated. The energy from a high-frequency generator is discharged via the electrode onto the surface of the container, which is inserted on the counter-electrode. In the discharge gap, bombardment of the material surface by the formed electrons and ions takes place.
For rotation-symmetrical containers this poses no problems. But for non-rotation-symmetrical containers it is not possible with the above described apparatus to maintain the width of the discharge gap constant without great engineering expenditure.
German OS 27 26 691 shows an apparatus with which it is possible also on non-rotation-symmetrical containers to treat the surfaces of the outer wall on the corona principle. But this apparatus has the disadvantage that because of the machining tolerances the gap width cannot be kept exactly constant over the entire circumference to be treated. This then leads to the result that the discharge occurs, not uniformly distributed over the entire surface to be treated and hence fluctuations of the surface tension on the container are possible.
Equally disadvantageous is the fact that for each container form and size a set of electrodes must be made, so that the cost of using this installation for different containers increases. From cup printing machines it is known practice to place the container to be printed on a rotation-symmetrical mandrel which is mounted freely rotatable on an axle and which has a small diameter relative to the inside diameter of the container. Parallel to it a mandrel is provided which is drivable and can carry the printing block, for example. By this arrangement the wall of the container is passed between the two mandrels, hence rolling off on the freely rotatable mandrel. Now if with this arrangement a corona treatment would be made possible by appropriate redesign of the mandrels, one would notice that the surface is treated also on the inside of the container, since in portions of the container where the curvature of the wall has a large radius by comparison with the radius of the mandrel the container wall makes contact practically only on a generatrix of the mandrel, and on either side of this generatrix, as the counter-electrode has a certain width, a gap is formed too, thereby permitting bombardment by electrons and ions. Due to this, more energy must be applied if nevertheless the surface of the outer wall is to be treated uniformly and in the desired degree. Moreover, treatment of the inside of the container is usually undesired.